Improvements of high-frequency electronic devices for consumer products such as cellular telephones, pagers, and wireless personal data assistants (PDAs), have given rise to a need for improved electronic testing. At the same time, pressures are applied to product manufacturers to reduce testing costs.
An important component in the testing of high-frequency electronic devices is the microwave synthesizer. Synthesizers are electronic instruments that generate test signals of variable frequency. The test signals are generally single frequency “tones” having low noise. Modern synthesizers include programmable electronics that afford them high frequency resolution over a wide range of frequencies. “Microwave “synthesizers” are synthesizers that produce output signals in the microwave frequency band, i.e., near 1 GHz or higher.
A common type of test for high frequency devices involves measuring the electronic noise that the device produces. To perform this type of test, the device under testing (DUT) is connected to a test system, or “tester.” The tester generally includes a microwave synthesizer. Noise on the output signal is then measured and the measured noise is compared with test limits to determine whether the DUT's noise performance is within the test limits.
For many high-frequency devices, the output signals from the DUT are generally a function of the input signals applied to the DUT. For example, if the input signal has a frequency fin, the output generally also has the frequency fin, or a multiple thereof. The exact input-output relationship depends upon the type of device being tested, but some numerical relationship between input and output is usually present. This being the case, any noise produced by the synthesizer may appear at the output signal. This noise creates an uncertainty in any noise measurement of the DUT, since it is not clear whether the noise measured is produced by the DUT or injected by the synthesizer.
Therefore, the noise of the synthesizer is one of most important aspect of its specification. By reducing this noise, measurement uncertainties are correspondingly reduced and the quality of testing is improved.